1. Field of the Disclosure
The present invention relates to a method of through-thickness reinforcing a laminated material with a reinforcing element. In particular, the present invention relates to a method of heating a laminated material to insert a reinforcing element.
2. Description of the Related Art
Laminated composite materials, in which reinforcing fibres are held within a polymeric matrix, are extensively used in many engineering applications. Such materials can generally provide a higher strength and stiffness per unit weight than conventional metallic materials. This makes such composite materials advantageous for weight sensitive applications, such as those in the field of aerospace.
A known problem with laminated composite materials is their poor inter-laminar, or through-thickness, mechanical properties in comparison to the corresponding in-plane properties. Such low inter-laminar strength and fracture toughness can constrain the design of composite parts and may even limit the use of such materials for certain applications.
One solution to this problem is the use of a toughened matrix material. Such matrix materials are generally significantly more expensive than conventional matrix materials, often have poor high temperature properties and may still not provide a sufficient increase in fracture toughness.
An alternative solution to improving inter-laminar strength properties is the insertion of through-thickness fibres into the laminated material. Various techniques have been developed for the insertion of such reinforcing fibres.
One method for inserting through-thickness reinforcing fibres into the laminated material is stapling or z-pinning. These reinforcing fibres are generally fibrous in structure and may be formed with a 45° chamfer at the insertion end. Through-thickness reinforcing fibres are intended to resist shear forces so as to improve the inter-laminar strength and fracture toughness.
The reinforcing fibres may be inserted into laminated material which is pre-impregnated with the matrix material. Once the reinforcing fibres have been inserted into the laminated material, the laminated material and matrix combination can then be cured in order to form a consolidated composite material. The composite material may then, with or without further processing, form composite components.
Inserting the through-thickness reinforcing fibres through the pre-impregnated material is easier, and causes less damage to the reinforcing fibres, if the matrix material has an appropriately low viscosity. The viscosity should be such that the reinforcing fibre can be inserted, but that the matrix material surrounding the layers making up the laminated material is retained.
FIG. 6, which is a rheology plot (dynamic viscosity vs. temperature) for a typical epoxy resin material that might be used in a laminated composite material, shows how this can be achieved by heating the pre-impregnated laminated material enough that the reinforcing fibres can be inserted through the softened matrix material. It can be seen from FIG. 6 that, by heating the laminated material, the dynamic viscosity of the epoxy resin will be considerably lower than at room temperature. This reduction in viscosity greatly aids the process of inserting the reinforcing fibres.
However, the heating process presents challenges as, usually, many thousands of reinforcing fibres are required in order to manufacture a suitably strong composite component.
One option is to heat the pre-impregnated laminated material just enough that the matrix material does not reach its gel point. The gel point is the temperature at which the curing process begins and the matrix material will begin to harden. The gel point is shown in FIG. 6 as the inflection point of the rheology plot, i.e. the point where the reduction in viscosity induced by heating ceases and further heating causes a rapid increase in viscosity as the material starts to cure or harden. Once begun, the curing process cannot be reversed.
Keeping the matrix material below its gel point may be achieved by cycles of heating and cooling the laminated material as the many reinforcing fibres are inserted. This need to keep the laminated material cool enough adds considerable time and complexity to the manufacturing process. It can also therefore, be a process which consumes a large amount of energy. Taken alone, the process of heating the laminated material so that the matrix material is uniformly softened can be time consuming.
Another option is to heat the pre-impregnated laminated material to a known temperature having a corresponding known and finite time-interval until the matrix material reaches its gel point. Once this temperature is reached, all the reinforcing fibres, which are to be inserted, must be in place before the matrix material begins to harden. It follows that, by reinforcing the laminated material in this way, there is limited scope for correcting production errors in the insertion process as once begun the curing process cannot be stopped.